Certain commercial steel compositions require relatively low amounts of carbon (less than 0.05%), nitrogen (less than 50 ppm), and sulfur (less than 30 ppm). In the past, methods of producing these low carbon and low sulfur steels used a combination of processes in a steelmaking furnace and a degasser. The prior method involved reducing the carbon levels in the steel composition in the steelmaking furnace, such as an electric arc furnace (EAF), making alloy additions during the tapping process to desulfurize and alloy the steel, and then shipping the steel to the degasser such as a vacuum tank degasser (VTD). This processing route was simple and quite straightforward.
To achieve the steel composition requirements of such commercial grades in the past, steel with very low carbon levels, such as less than 0.025%, was tapped at the steelmaking furnace. The dissolved oxygen levels associated with these low carbon amounts were in the order of 1200 ppm to 1400 ppm in the furnace before tapping. Where the degasser was a distance from the furnace, the steel was tapped at approximately 1700° C. to compensate for temperature losses during transportation to the degasser. During the tapping process, the steel was deoxidized with aluminum and ferrosilicon (FeSi). Lime and aluminum dross were also added to create a fluid, deoxidized, desulfurizing slag. By these additions, the desulfurizing reaction was started in the ladle during shipping to the degasser. At the degasser further additions of aluminum, lime, calcium aluminate and dolomitic lime were made to ensure desired sulfur removal during the degassing cycle.
The prior process had drawbacks, including high refractory wear on the steelmaking furnace. The elevated tapping temperatures and high oxygen content required before tapping the steelmaking furnace had an adverse effect on productivity at the furnaces. The high temperatures and high oxygen conditions enabled high amounts of FeO in the slag at the high temperatures, causing excessive refractory wear on the furnace walls. This led to increased furnace down-times while the furnace refractories were patched with gunite. Also the high FeO content in the slag results in lower efficiency in steelmaking as more iron units are lost in the slag.
The prior process also required the use of low carbon alloys and additives throughout the subsequent processes from the steelmaking furnace to maintain the low carbon level below 0.05% by weight. Low carbon alloying elements, such as low carbon FeMn, were required to provide desired elements without upsetting the final carbon content of the steel. Recently, the price of low carbon ferro-alloys has increased significantly, making this method economically undesirable to produce such low carbon steel. Further, lowering the amount of carbon in the steel composition in the steelmaking furnace required additional decarburization time, which also adversely affected productivity at the steelmaking furnace. Cost was further increased as a result of more silicon and aluminum required to deoxidize the steel composition as a result of the higher oxygen content.
Additionally, in prior decarburizing and desulfurizing processes, aluminum has been the primary deoxidant. In certain applications, aluminum is not desired in the steel product, requiring additional compositions and processes to retain the aluminum in the slag. There remains a need to decrease production costs of low carbon, low nitrogen, and low sulfur steels.
We have found an alternative method of making a steel with low carbon less than 0.05% by weight that avoids the need for aluminum additives, reduces wear on refractories, and increases steelmaking efficiency.
Disclosed is a method of desulfurizing a silicon killed steel including the steps of:                forming a slag over a molten metal,        drawing a vacuum to less than 5 torr over the combination of slag and molten metal,        stirring the molten metal and slag,        deoxidizing and desulfurizing the molten metal and slag to degas the steel reducing at least sulfur, nitrogen, oxygen, and hydrogen contents, and reducing activity of oxygen in the molten metal to less than 30 ppm, and        forming a slag composition after degassing the steel comprising:                    CaO between about 50 and 70% by weight,            SiO2 between about 20 and 28% by weight,            CaF2 between about 5 and 15% by weight,            MgO not more than 8% by weight,            Al2O3 not more than 1% by weight, and            a combination of FeO+MnO not more than 2% by weight,            where the sum of CaO+CaF2+SiO2+MgO is at least 85% by weight.                        
The step of drawing a vacuum may include drawing a vacuum to less than 1 torr.
The stirring step may additionally reduce lead, zinc, bismuth, antimony content in the molten metal composition.
The stirring step may involve bubbling inert gas at a rate between 0.1 and 1.0 SCFM per ton of molten metal in a heat for between 10 and 40 minutes. The stirring step may reduce the sulfur to less than 30 ppm, and alternatively less than 10 ppm, nitrogen to less than 50 ppm, activity of oxygen less than 15 ppm, and hydrogen less than 3 ppm.
Alternatively, a method of desulfurizing a silicon killed steel may include the steps of:                forming a slag over a molten metal,        drawing a vacuum to less than 5 torr over the combination of slag and molten metal,        stirring the molten metal and slag, and        deoxidizing and desulfurizing the molten metal and slag to degas the steel reducing at least sulfur, nitrogen, oxygen, and hydrogen contents, and reducing activity of oxygen in the molten metal to less than 30 ppm, and        forming a slag composition after degassing the steel comprising:                    CaO between about 50 and 70% by weight,            SiO2 between about 20 and 28% by weight,            CaF2 between about 5 and 15% by weight,            MgO not more than 8% by weight,            Al2O3 not more than 1% by weight,            Cr2O3 not more than 15% by weight, and            a combination of FeO+MnO not more than 2% by weight,            where the sum of CaO+CaF2+SiO2+MgO+Cr2O3 is at least 85% by weight.                        
Also disclosed is a method of desulfurizing steel including the steps of:                (a) preparing a heat of molten steel composition in a steelmaking furnace to a tapping temperature as desired for desulfurization at a vacuum tank degasser,        (b) tapping open into a ladle the molten steel composition with an oxygen level between about 250 and 1200 ppm,        (c) providing a slag forming compound to the ladle to form a slag cover over the molten steel composition in the ladle,        (d) transporting the molten steel composition in the ladle to a vacuum tank degasser,        (e) decarburizing the molten steel composition at the vacuum tank degasser by drawing a vacuum of between about 0.5 and 300 torr,        (f) after decarburizing, drawing a vacuum of less than 5 torr and adding flux components, deoxidizers, and alloying agents forming a slag composition having less than 1% Al2O3 to the molten steel to deoxidize and desulfurize the steel, and        (g) stirring the molten metal and slag composition deoxidizing and desulfurizing the molten metal and slag composition to degas the steel reducing at least sulfur, nitrogen, oxygen, and hydrogen contents, and capable of reducing activity of oxygen to less than 30 ppm.        
The step of drawing a vacuum may include drawing a vacuum to less than 1 torr.
The step of decarburizing the molten steel may be between 2 and 10 minutes.
Depending on the final chemistry, the method may further include the step of prior to the step of decarburizing, adding an additional oxygen source.
The step of adding flux components, deoxidizers, and alloying agents may include providing an initial slag composition prior to deoxidizing adapted to provide a slag composition after degassing the steel comprising:                CaO between about 50 and 70% by weight,        SiO2 between about 20 and 28% by weight,        CaF2 between about 5 and 15% by weight,        MgO not more than 8% by weight,        Al2O3 not more than 1% by weight, and        a combination of FeO+MnO not more than 2%,        where the sum of CaO+CaF2+SiO2+MgO is at least 85% by weight.        
The step of adding flux components, deoxidizers, and alloying agents may include providing an initial slag composition prior to deoxidizing adapted to provide a slag composition after degassing the steel comprising:                CaO between about 50 and 70% by weight,        SiO2 between about 20 and 28% by weight,        CaF2 between about 5 and 15% by weight,        MgO not more than 8% by weight,        Al2O3 not more than 1% by weight,        Cr2O3 not more than 15% by weight, and        a combination of FeO+MnO not more than 2% by weight,        where the sum of CaO+CaF2+SiO2+MgO+Cr2O3 is at least 85% by weight.        